Current-limiting fuse



Sept. 22, 1953 F. J. KozAcKA CURRENT-LIMITING FUSE 4 Sheets-Sheet lFiled Jan. 30. 1951 ab 6*"ac/J a si Sept. 22, 1953 F. J. KozAcKACURRENT-LIMITING FUSE 4 Sheets-Sheet 5 Filed Jan. 30, 1951 RsouceocRoss-sscrlou E ....m T SRI' YE P D A EE TT s Ts TIME (M|cRosEcoNos)-SELECTIVE SYSTEM Sept 22, 1953 F. J. KozAcKA 2,653,203

CURRENT-ummm@ FUSE Filed Jan. :50. 1951 4 sheets-sheet 4 W x i f.'

' l l l l I l I l I 1 l I *v un |559@ cunRsNT Fv'ghg 35%6 RENT looc loo

0') D Z 8 u |c lll E D AE z Ill l I 5 .ool v o lpoa 2,000 3,000 4,000spoo LeT-mnu cunnenr- Penceu'r or RATING Patented Sept. 22, 1953 tions'which virtually" prevail A111i ofthe? Ytxrne.

tatnge'IectcaI macTnery, fappratus" and v ices Whichere .subjected tohigh mech stresses @nstrains but-durin relative simesgyet'thisis onlyrami A. which' is 'inhereritlyfnsolind sllncert 'consists' in that l the;mechanical' prQp-rtiespf -r'o'ta'fcin elecmeal -machineryaartgs and dedetermined byth A mntsland rel yely .sxnall by abrmal'"faulticdnditions.D

,tncerrotem advent of the current-limiting fuse which made it possibleto interrupt short-circuit currents extremely rapidly at relatively lowvalues, i. e., before they reached the highest values which the circuitis capable of producing. This enabled a drastic reduction of theshort-time ratings of rotating electrical machinery, electricalapparatus and devices.

Heretofore it was necessary to design rotating electrical machinery,apparatus and devices, even if protected by current-limiting fuses, insuch a way as to render them capable of withstanding the relatively highlet-through currents prevailing if the fuse was designed for highminimum fusing currents, i. e., if it had a high current carryingcapacity. This limitation resulted from the fact that thecurrentlimiting fuses which were known prior to this invention had afixed or inflexible ratio between their minimum fusing current or ratedcurrent and their let-through current which was relatively small andcould not be increased. This ratio and its inflexibility for anyprototype of current-limiting fuses made it impossible heretofore totake full advantage of the potentialities inherent in thecurrent-limiting principle. In other words, it was` still necessary todesign much more mechanical strength and thermal shock resistance intoany piece of electrical equipment than is warranted by its normal opierating conditions and reasonable safety factors.

It is, therefore, an object of this invention to provide a protectivedevice in the nature of a current-limiting fuse permitting a drasticreduction of the mechanical strength and thermal shock resistancerequirements compared to those which had to be imposed heretofore.

Another object of this invention is to provide means enabling electricsystems and any type of electrical equipment associated therewith to bedesigned on a more economical basis than was possible heretofore.

Another object of the invention is to provide means enabling the use ofelectrical equipment having smaller short-time ratings than wererequired heretofore under the same or similar conditions, and enabling areduction of the shorttime ratings even though the availableshortcircuit current of the circuit in which the equipment is intendedto be used is considerably increased.

Another object of the invention is to provide a type of current-limitingfuses which is designed in such a way that the let-through current canbe determined independently, or substantially independently, from theminimum fusing current and the rated current thereof.

Another object of the invention is to provide a type of current-limitingfuses which enables an increase of the ratio of minimum fusing currentto peak let-through current compared to prior art current-limitingfuses.

Another object of the invention is to provide a type of current-limitingfuses which makes it possible by minor changes in design to vary theratio of rated current to let-through current.

Another object of the invention is to provide an improvedcurrent-limiting fuse which is particularly suitable for the higher endof the low voltage range, i. e., the range around 1000 volts.

A still further object of the invention is to provide a current-limitingfuse of very small overall size suitable for the highest availableshort-circuit currents that may occur in present day practice and inforseeable future practice.

In carrying out my invention I prefer to us fuse links of whichsubstantially the entire length, or at least the preponderant portionthereof, consists of a variable cross-sectional area section having arelatively large number of serially related portions of relatively smallcrosssectional area and intermediate portions of relatively largecross-sectional area.

Fuse links of this kind make it possible to distribute the thermal dutyof the arc quenching filler relatively evenly over the entire fillerbody. This in turn tends to permit a reduction of the overall size ofthe fuse and more particularly of the internal space of the fuse casing.Fuse links of the above kind have a relatively even longitudinaltemperature distribution and are therefore capable of forming rapidly agap of the length required for interruption or a number of small gaps inseries having the same effect. This in turn tends to limit the arcingtime and the arc energy and makes it possible to limit the internalspace of the fuse casing, or the required amount 0f arc quenching ller,respectively. Another factor tending to permit a decrease of the size offuses according to this invention consists in the possibility ofpredetermining the let-through current irrespective of the minimumfusing current, thus enabling to limit the interrupted power on theoccurrence of fault currents of short-circuit current proportions to anydesired minimum value. The heat absorbing capacity and the volume of thearc quenching filler may be limited in proportion to the limitation ofthe interrupted power. The combined result of all these features is adrastic limitation of arc energy on interruption of small protractedoverloads as well as on interruption of fault currents of short-circuitcurrent proportions and this makes it possible to provide acurrent-limiting fuse having much smaller dimensions than any comparablecurrent-limiting fuse known or proposed heretofore.

For a better understanding of my invention reference may be had to theaccompanying drawings wherein:

Fig. 1 is a substantially central longitudinal section through a singlefuse constructed in accordance with my invention. Certain parts of gig.1 are shown in elevation rather than in sec- Fig. 2 is a transversecross-sectional view of the fuse shown in Fig. 1 taken along the line 22 thereof;

Fig. 3 is a front view of a fuse link intended for fuses of the generaltype shown in Figs. 1 and 2;

Fig. 4 is a substantially central longitudinal section through acomposite fuse which cornprises a plurality of fuses of the kind shownin Figs. 1 and 2. Certain parts of Fig. 4 are shown in elevation ratherthan in section;

Fig. 5 is a transverse cross-sectional view of the fuse shown in Fig. 4taken along the line 5-5 thereof;

Fig. 6 is a diagrammatic representation'illustrating the mode ofoperation of fuses according to this invention in comparison to the modeof operation of prior art current-limiting fuses;

Figs. 7 (a) and 7(1)) are two diagrams which illustrate the behavior ofa fuse embodying the invention under normal load conditions;

Figs. 8(a) and 8(b) are two diagrams which illustrate the behavior of afuse embodying the S invention under:ifaiiltticutrentsfloIfJshort-cmcuitcurrentproportions;

Fig. 91- is' the maxim-cmiletzthrough 'current versusavailableshort-circuit current characteristi'c of `a p'rot` `typepriorrartllcurrent-limiting fusea-nd of a currentl-lirhitin'gifuse'ottheeameprototype changed; howe'ver, accordance Ywith this invention; I,'liglyl10 illustrates aif'amily o'f cuives referring to faicertaixitype orfpror v"art current-limiting fuses. igure shows'lthel'total clearingtime in seconds plottedfversusi'let-though current in amp'eres.l Boththe'fab'soissaeiand the ordinates are drawn on -a logarithmic' scale;

Fig. l1l. 1shows a-"aniilyofcurves of a kind similar to that-shovfninil'glfl' -bu't-'Fig ll're'fers to a l.group of ifuses:havin'gilthe'fsame ratingf vyet diiferingas'regardsflet-'thr'oghcu-rrentj and Fig'. 12 showsf-twoltotal clearingftimeversus let-through 'current characteristics' of which one refersto apriorfar-t fuseand the other-fte the s'am'efus'eprovided, howeverf'withthelet-through current control feature-according to' vthis Ainvention.

13 is a diagammatic representation of a protective systemr'eifrliboitlyin'g the invention and comprisinga pair of `seriallyconnected fuses having thesame cu'rrentrating,y yet operatingvselectively on the occurrence of high faulticurrents.

Lik'e reference signshav'fe been applied througheutaflliigures!te'-:desiign'ate like parts.

lRefei-ringf-novv to'tFi-gl. 11a-3nd 2, reference numeral "Ifhas been'appliedfto'ia-casing of laminated insulatingl material Vin Whichfr theribb on-type link 2 ofsilver is mounted. lhe"flink 2-mightbe made ofa"metalfothei'"-than silver, however, forY the reasons`'staltedvabve'sil'ver is'preferable. Link 2' is provided with aApluralityof equidistant circular perforations orIl-ioles '2a. Casing Ilis iilledl inpa-rt with a-suitable vsand-'filler 3. Clean chemicallypure Vquartz sa-nd is the preferred filler` material f vfor thifstyp'e fof fuses.l The right end of casing l is filled with a-gas`evolving-pulverulent l materia-l`^3a` such' asi Achemically pure chalk.1The pressure generated -bythe chalk fillervauhderfthe heat of the arcremains Within reasonable lim-itsfsince only ai relatively shortlengthofilink 2 is-surrounded by the gas-'evolving 1le`r- 3a; TheVincreaseoffpressure 5kwithincasing l'du'eto the evolution of gasfronller 3a resiiltslin an-increase of the dielectric strength of thespace Within Vthe-fuse ln accordance with Paaschens lawf Thatportion oflink 2 which is surrounded by `the gas evolvingY ller 3a maybe providedwithf an alloy-forming element 5 in the form-of atin rivet; Ti'nha's alower Vfusing point than silverand forms -an alloy Withsilver which hasa lower fusing Apoint than silver. If tin rivet 5 4is insertedin one of`the holes -2a, fusion will generally be initiated on the occurrence ofsmall protracted overloads atthe point where the rivet 5 is situated.If, on the4 other hand, rivet 5 is omitted, fusion -Will generallybeinitiated on the 'occurrence of s'mall protracted overloads at therestricted crossesectionportion y 2c, V2c formed adjacent the pair ofcircular -`holes Zaat the center vof the link AIt -Will'bev observedthat each restricted cross-section portion 2c, 2`c"consists of twocurrent'paths inl'par'allel. IUpon -vaporization of thesetW`o=parallel"currentpathsA the gaps thus formedare 4occupied by "apaiof smalll arclets'iin parallel. 'Since arclets' in =parallel areunstable; one r-arolet willi extinguish 'Y very "rapidly,V The other'arclt willfllo'w 'course Within ai very shorlt-tiine.A f

-1 iIt is'importantito avoidfmixing ofzll'ers :arend :lai 'Whenlthe fuseischipped orhandledand, therefore, a partition llis. arrangedbetweenllers Bfand 3a. v l' It `will `Vbe noted l'tha'tftheiin't'erna'lidiameter of `easing 'I 'is relatively-small, i.e.', itTis inf-'fthe range.l i of twice the` '-width off-the -lf'se flink rllhisfreductionorsize'isfdue .to the `faetitliatithe fuse is designed toilimi-t arc energy -I more fdrastically than:any:current-riimitingzfuse'Lknownor proposed heretofore, and'moreApa'rtic-:ultufly` toi-the l feature lwhichv will 4'be Jdescribedbelow @enabling aireduction` of the let-'throughcurrentffbelow theminimum vvalues which were thought heretofore to=V be thelowest'pos'sible'limit. 'Onrlthe fith'er hand, thethicknessof'2thewal1fofIthe -ce'ising Il is-considerable to provide themechanical strength r'iecessary because ofthe generation of airelatively4high amount of VVVpressurefftherein. f Bth ends ofA thelfuse casingare5clci'sed;lbyl'i',eixn"ix`ral caps or ferrules' indicated-'byreferencefsign'sa and 6b! Fuse link 2` is conductivelyfco'nnected tothecaps or ferrulesa, Sb'ib'y suitableimeans such as, for instance, spotWelds. Ferru'les 6a, 6b are rolled intothe casing I 'for .addedistrengthlluselinl'r 2 is also-provided Withia-prltionlof reducedvcross-sectio'rlr-"Zb.` The portionrof reduced cross-'section- 2b issmallerinfcross=sectiorithan thesum ofthe cross-'sectionsof-tlieiparallel current paths 2c, 2c formed 'laterallyffadjacentleachhole l2a.` The length df-thefporltionwof YArieduc'iedcross-section 2b isrelativeliyshort-and this lirhits the amount of heat which is beinggenerated' at this point Since thefge'ome'tryof.theileft side of thelink 2 is dlierentfrorni the geometry 'lof any point at lthe right side.thereo'ffseel'flig. i1) the thermal characteristicso'f bthli-nk'sectin'swill Abe diiierent. 'The -rninlimum-ffu'sirrg current is mainlydetermined by-ltheii'ifghtsideof fthe link. Assuming that 'rivet '5 hasbeenlo'iiiitted, then fusion Will be initiated-'fon-`the!-oceurreric'e'of small protratced overloadszatthe-pointf reducedcross-section f2s, "2c at the center oflilnk '2. *The reducedcross-section portion -2b is vformed `bri/two lateral juxtaposed squarenotches 'LIJ'he-igeom# etry oflthe entire' link andlthedesignoi'ithe-parts associated with itareadetermined inlsuch a'lwaytha-'tthe'ratiol Ofthe ratelofheat vgenerfatior'i to the irate' of heatdissipation is'lllarge for Yeach reduced cross-sectionportiononsistinglof air of parallel current `paths 2c, l2`c-, than forhe neck'portionV lzb -oni thefoccurre'nce'lof Ycurrents having a'rel'ativelyVsmall rate? 01E-frise Aand larger for said neck portion-"2b than saidreduced-*crosssection portions Y 2 c, 2c on the occurrence ofvercurrentshaving afrelatively rapid V-rateoii'rise. If -there is a smal-lprotracted"overloadtheira-4 tio of heat generationtoheatl-iss'ipationlwillibe relatively close-to unity at'the neck?2li-lie ause of the relatively short length oftheisamefnd-lbecause `ofthe! relatively large Ametal-I surfaces and masses "immediatelyadjacentthereto, `including the large cap-6a. A"Hence the-rise' dfftempefatureatthe neck portion-l2b vvill'beslOwS-eorpare to that at the`restricted?cross-sectional areal portions 2c, 2c Where in case of smallprotracted dis'sipationdiffersinore from- 'u-n'ity than'y at the neck"2b. "Consequently, -fusinga'nd tai'in'g will be-'initia'ted' at thehottest of theserially'4 related restricted 1' cross-section fp'ort'ions2c, y2c .if -I'and when the v.overload ifs small but 'Iprotracted Vsuchas,#for'-instance,hardlyI exceeding the :minimum fusing' current. z s

:.fThe behavior of the-fuse fis, however,I *very different if the faultcurrent is high and the rate of its rise high. A typical case of thatnature would be the occurrance of a short-circuit current. The geometryof the serially related reduced cross-section portions 2c, 2c and of theportions of link 2 immediately adjacent said reduced cross-sectionportions of link 2 and the geometry of the neck portion 2b and of theimmediately adjacent portions of link 2 are determined in such a waythat the rise in temperature at the neck portion 2b is much more rapidthan at the reduced cross-section portions 2c, 2c if the fuse issubjected to fault currents of shortcircuit current proportions. Therise in temperature at the neck portion 2b will be so rapid as to causefusion of the neck portion 2b in a small fraction of a half cycle of thecurrent wave if the fuse is used in an A. C. circuit. If the fuse isused in a D. C. circuit fusion of the neck portion will occur in acomparatively short time upon initiation of the short circuit. In bothcases arcing will last less than 1/120 second. In the A. C. case and inthe D. C. case the interrupting time will generally be in the order of afew milliseconds, e. g., 2-10 milliseconds. The interrupting time is theinterval betwleen the fault initiation and arc extinction. The currentrequired to fuse the neck 2b is the let-through current of the fuse.Vaporization of neck 2b acts like a trigger resulting in almost instantvaporization of the preponderant portion of the length of the linksituated to the right of neck 2b as seen in Fig. 1.

It follows from the foregoing that fuses according to this invention areprovided with a unitary fuse link 2 consisting of a strip or ribbon ofsheet metal which has two dissimilar portions of reduced cross-section.The relatively wider of said portions determines the minimum fusingcurrent while the relatively narrower of said portions determines thepeak of the let-through current. The relatively narrower link portion 2bfuses in considerably less time than 1/120 of a second, generally inless than half a millisecond, on occurrence of fault currents in thenature of short-circuit currents because its heat dissipating ability isextremely small or negligible if the rate of rise of the fault currentis high. The relatively narrow link portion 2b of the fuse link 2remains at a lower temperature than the other portions 2c, 2c of reducedcross-section on the occurrence of fault currents of the same order asthe minimum fusing current because its heat dissipating ability isextremely high if the rate of rise of the fault current is small, or thefault current is small. If the fault current `is relatively small andtin rivet 5 is omitted, arc initiation will occur at the reducedcross-section portion 2c, 2c situated at the center of link 2. Fusion ofthe other reduced cross-section portions 2c, 2c will soon follow. Theformation of a plurality of arclets in series effects more rapidinseration of arc resistance into the circuit than can be achieved byarc initiation at one point only and, therefore, tends to limit thearcing time and the arc energy.

If the fault current is relatively small and tin rivet 5 is in place,arc initiation at small protracted overloads will `occur only at thepoint where the rivet `l is situated. The arc gap thus formed will growto the left and right, as seen in Fig. 1, preponderantly to the leftsince that portion of the link 2 is relatively warmer. The fact that thefuse link comprises a plurality of points of reduced cross-sectionalarea 2c, 2c

situated adjacent the point of arc initiation causes more rapidformation of the length of the gap required for interruption of thecircuit, since each of said points of reduced cross-sectional area 2c,2c is at a relatively high temperature at the time when `the arc isinitiated at the point where rivet 5 is located. On account of thepre-heating of all points of reduced cross-sectionalV area 2c, 2c arelatively small amount of arc energy is required for vaporization of asufcient length of the link to eiect nal circuit interruption.

Considering a fuse according to this invention having a link without tinrivet or similar means for local reduction of the fusing point, there isa critical value for protracted relatively high overloads at which, andabove which, fusion of the link is initiated at the narrow neck portion2b rather than at the reduced crosssection portion 2c, 2c situated atthe -center of the link. Similarly there is a critical rate of rise of:current at which fusion is initiated at the neck portion 2b rather thanat the reduced cross-section portion 2c, 2c situated at the center ofthe link. At protracted overloads which are below the critical overloadvalue at which fusion is initiated at the neck portion 2b initial fusionmay occur at a reduced cross-section portion 2c, 2c.

A similar shift of the point of initiation of fusion may be observedwith fuses according to this invention having links which are providedwith a tin rivet or a similar means for local reduction of the fusingpoint. A fuse according to Fig. l the link of which had the dimensionsmore fully stated below in connection with the description of Fig. 3 wasfound to have a minimum. fusing current of 20 amperes. If subjected tocontinuous currents in the order of 20 amperes, fusion would beinitiated at the point where rivet 5 was arranged. Upon increase of theoverload to 40 amperes, initial fusion would occur at the neck 2b ratherthan at any of the portions of restricted cross-section 2c, 2c.

Referring now to Fig. 3, the ribbon-type fuse link of silver which isshown therein comprises eight series holes 2a each having a diameter of.046 inch. The centers of holes 2a are spaced .203 inch apart. The totallength of the link is 3%. inches, its width .086 inch and its thickness.U03 inch. Upon mounting of the link in the casing of the fuse thelength of the link is reduced to 2 inches. The length of the neckportion 2b which determines the let-through current is .O31 inch `andthe width thereof :010 inch. The ratio of the cross-section of the neck2b or its width, respectively, to the cross-section of the fuse link atits point of largest width, or to its width at these points,respectively, is in the order of 1:8.6 and this ratio proved to berather critical. The distance between the left end of the link and thecenter of neck 2b and the distance between the right end of the link andthe last hole 2a farthest to the right side of the link are each 11%inch. The distance between the center of neck 2b and the center of thehole 2a farthest to the left side of the link is .203 inch. The ratio ofthe cross-sectional area of neck 2b to the cross-sectional area of eachpair of current paths 2c, 2c is 1:4.

`The metal parts 2b', 2b formed by link 2 are arranged immediatelyadjacent the neck portion 2b and at opposite sides thereof. The metalparts 2c', 2c are arranged immediately adjacent each reducedcross-section portionZc, 2c

amazes 9v Y and' at opposite sides thereof. The cross-sectional area oflneck portion'. Zbdetermines the let-through current of the fuse in whichlink 2 is mounted. The cross-sectional area of the adjacent linkVsections 2b', 2b' and their length and the parts with whichithey `are incontact, including terminal. cap. 6a, cause a relatively rapid andintense cooling of theV neck portion 2b as long as the link-lissubjected .to a relatively small overload. In spite of the` fact thatthe cross-section ofthe reduced cross-section portions lcy 2cisfour-.times as high as the cross-section of-neck portion. 2b; thetemperature of the former portion rises more rapidly on the occurrenceof. small protracted overloads on account of the fact that.theintermediate link portions-202 2c are'. relatively ineffective coolingmeans since eachintermediate link portion 2c', 2c is heated by tworeducedcross-section portions 2c, 2c.A The-axially outer spot heatedportions of link2 "form eiective thermal insulators of the axially innerportion of link..2 and on account of this the temperature of the latterportion follows rapidly changes. of current intensity within the lower.overload range. In other words, the fact that fuse link 2 isprovidedwith a plurality of reduced cross-section portions 2c,2c,.reduces the temperature `gradient between the centerl of the linkand the adjacent axially. outer partsthereof and the smallness of saidtemperature gradient greatly increases the rate of `change oftemperature by which the centerl portion of the link responds to changesof current intensity Within the overload range. of reducedcross-section.2c,.2c where the temperature of the link 2 'is highest, ifa fuse of which link 2 forms part is subjected to long protractedoverloads such as for instance, overloads in therange of the minimumfusing cur- .f

the; heat, generated-.at thew reduced .cross-section portions 2b and 2c.2ci andzthe heat. :absorbing ability.v of the adjacent portions 2b. 2band 2c', 2c and. of any other parts of .the fuse structure does not comeinto: play. Consequently? among all Aportions of reduced cross-.sectionZb and 2c, 2c thatwillzmelt soonestwhich has the smallestcross-sectional area. Thisis the neck portion 2b. It is of. considerablefimportance-f to properly choose the average grainsizeof thequartzllerforlling the individual fuse units. The flow of hot. metalvapor throughthe interstices between the quartz particles is obviously highlyturbulent and thisr ltendsf t`o-i increase the Irate of -deionizationof?Athe arcproducts but tends to2 increasev iloveresistance?` Quartz-sandof-30/4o'meshsizel proved-to hemost satisfactory in fuses` according-to:thisinvention;

Referring/howto Figi 6, linevIr -indica-testhe rated current-of a givenlcurrentlimiting fuse and-Ir max thepeak value of Ir. Line I indicatesa` completely asymmetrical short-circuitl current. Theshort-circuit isinitiated` atv the time` Iueandfitsinitial rate oirise has: beexrindicated' byl drawing.A a. tangent to glinev I throughl point To." If' aprior art current-limiting fuse Hence arc initiation occurs at the pointf 10 isprovided in. the circuit, bothl the maximum-.letthroughrcurrentand the" ratedy current `andalso the minimum fusing current aredetermined by the .cross-"sectional- 'area of' the point rof smallestcross-section;ofiv "the fuse' link.: The-rated. current isdeterminedsby'the prototypeof fuse under consideration' including.manyii design# factors among 'whichy the -f crossesectiona-lf area otthe point 'oft smallest cross-sectionA is one: The lmaximum:let-through.Y current depends only uponA the point Yof:asrrrallestffcross-isection. since heat exchange phenomena have"l very little,Vand=1p1-ach tically nas-part in'the heating offthe pointe'ofsmallest'lcrosseseetionsf There" isf, therefore, a iixed relationbetweenratedcurrent andy maximum .let-through fcurrent; i. e., anychange'of thezone affectsstherothenf To'be more specific, anyincrease'of the .rated current resultsiin an increase1 o1'. themaximum-"letethrough current', thusv precluding to. increase 'one and.reduce the other. This appliesto fuses of the type wherein both the.interruptionV ofv relatively high "currents and of relativelyflow'fcurr'ents'is initiated by fusion of a unitarylink andto fuses withacorn-'- posit'e fuse link including va 'section ofi 'metalV hav'- iriga relativelylo'w melting point wherein interruption of relatively lowcurrents is initiated by fusion of the Alow meltingpoi'ntsection. In the'latter type of fuses a-portionor` portions -ofreL duced cross-sectionserve togenerate atleast in part the heat'requir'ed for fusing the lowmelting point section,v and, therefore, in fuses of that kind the ratedcurrent and minimum fusing current and 'the let-through current are#likewise in a xed relation to each other.4

In Fig. -6 line I1 "indicates the let-through current which is` allowed'to pass through aprior art current-limiting fuse wherein let-throughcurrent and rated-'current `'fand also minimum fusing currentlf are`interdependent and any change of one of them has a marked effectupon theother. Line lrrefersfin particular" to a current-limiting fuseofthe'type wherein anyl increase of thef'rated'fcurrent (and minimumfusing current) results in a correspondinglyI large increase of" Ithelet-through current. The 'letthrough current` Irrises' from To to T2 insubstantiallythe vsame way as the available shortcircuitA current I..Atv T2 the let-through current begins to; decrease IromitsV peak valueI1 max to zero; Actual current zero is reached about the time Tapriortothe ilrst natural zero of the current wave followingfaultinception. In a fusel according. .to this invention having the samerated 'current Irtlie -fuse link is completelymelted at Ya `much earliertime and. a gradual reduction of Athe let-through current from its peak..value i1 maxto Zero begins at the time T1.

Actual current zero is reached aboutthe time T, generally prior tov thattime.

It is apparent that 1:1 max I1 max and this enables to include into'.the circuit rotating 4electrical machinery,r apparatus and devices.which have much smaller short-time ratings thanwas possible heretofore.The' rotating electrical machinery, apparatus and devices which are`applicable, if protected by .fuses according to this invention.. areunable to withstand'. impulse. currents of the peak value I1 mx andduration. Tdi-T2. andV therefora. cannot beused `where merely prior`artcurentlimiting fuses with their fixed relationship between maximumlet-through current and rated current (and minimum fusing current) arerelied upon for protection.

Since the arc energy which is dissipated during the arcing time is)t2-Pdtand since the vlet-through current values in the case of theprior art current-limiting fuses are much higher than incurrent-limiting fuses according to this invention, it is apparent fromFig. 6 that fuses according to this invention result in a substantiallimitation of arc energy and consequently a substantial reduction of thebulk of the cooling means required for dissipating the arc energy and,therefore, in a corresponding reduction of the overall size of the fuse.

Referring now to Figs. 7(a) and '7(b), these two diagrams show thetemperature of the neck portion 2b and the temperature of one 0f thereduced cross-section portions 2c, 2c near the center of the linkplotted versus time. time 0 the neck portion 21) and the reducedcross-section portion 2c, 2c are at ambient temperature. Upon closing ofthe circuit at the time 0, the temperatures of portions 2b and 2c, 2cbegin to rise. The rises of the temperatures of both follow differentexponential curves. Though the cross-sectional area of the neck portion21) is considerably smaller than the cross-sectional area of the reducedcross-section portion 2c, 2c under consideration, the rise intemperature of the former is smaller than that of the latter. It 'takesalso more time for the neck portion 21) to reach its steady statetemperature than for the reduced cross-section portion 2c, 2c underconsideration to reach its steady state temperature. It has been assumedthat the current to which the fuse is subjected is of permissiblemagnitude. Hence the temperature of the fuse link remains under thefusing temperature level which has been indicated by dotted lines fparallel to the abscissa.

The steady state temperature of the neck portion 2b is obviously lessthan the steady state Itemperature of the reduced cross-section portion2c, 2c.

Assuming now that the load current would be increased, this would resultin an increase of the steady state temperature of neck portion 2b and ofthe reduced cross-section portion 2c, 2c. In spite of its smallcross-sectional area the rate of rise in temperature of the former wouldbe less than that of the latter. If the increase of load current issufficiently high to cause fusion of the link, the fusing temperaturewill be reached first by the reduced cross-section portion 2c, 2c andthe neck portion 2b might never reach the fusing temperature of themetal of which the fuse link is made.

In drawing Fig. 8(a) and 8(1)) it was assumed that the fuse wassubjected for a considerable period of time prior to and at the time t=0to the same load current as in the case shown in Figs. 7(a) and 7(1)).Consequently, the neck portion 21) and the reduced cross-section portion2c, 2c will be at their respective steady state temperature equal to thesteady state temperatures shown in Figs. '7(a) and 7(1)). It is assumedthat a fault current of short-circuit current proportions is initiated:at the time Ts. Consequently the temperature of both the neck portion2b and the reduced cross-section portion 2c, 2c will then rise at a veryrapid rate indicated by the temperature versus timel characteristics ofFigs. 8(a) and 8(1)) wherein time is expressed in terms of microseconds.The neck portion 2h is At the cooledmuch more effectively than thereduced crosssection portion 2c, 2c. Yet, nevertheless, the rise intemperature of the former is much more rapid than the rise oftemperature of the latter. This is due to the fact that heat generationat the neck portion 21) occurs at a more rapid rate than at therestricted cross-section portion 2c, 2c under consideration and that therate of heat generation at 2i) and at 2c, 2c is so rapid that theirtemperature is virtually unaffected by heat exchange phenomena whichwould take more time in order to have any significant effect 'upontemperature distribution along the link. After the interval Atifollowing initiation of the fault current the fusing point of the neckportion 2b has been reached and the interrupting process proceeds fromthere on in normal fashion. If the temperature of the reducedcross-section portion 2c, 2c would continuously rise at its initialratc, reduced cross-section portion 2c, 2c would reach its fusing pointafter the interval Ata.

Actually it takes more time for the reduced cross-section portion 2c, 2cto reach fusing temperature. This is mainly due to the fact thatvaporization of the fuse link begins at the end of the time interval Atlresulting in a considerable increase of the resistance of the currentpath through the fuse and a concomitant decrease of current flow.Because of that decrease of current flow the rate of increase oftemperature of the reduced crosssection portion 2c, 2c decreases at theend of ntl and it takes Ata rather than Ata for the reducedcross-section portion 2c, 2c to reach fusing temperature.

It is of considerable importance with regard to the desired operation ofthe fuse that the neck 2b which determines the maximum let-throughcurrent be arranged immediately adjacent one of the caps or ferrules 6a,Gb (see Fig. l) rather than relatively close to the center of the fuselink 2. If the neck 2b is arranged relatively close to one of theaxially outer ends of the fuse link, the cooling of neck 2b is increasedon account of the relative nearness of the large mass of the terminalcap 6a and the relative remoteness of the additional points ofrelatively large heat generation formed by the reduced cross-sectionportions 2c, 2c. The cooling action of cap Ba upon neck 21) varies inaccordance with ambient temperature conditions but is always so large asto keep the neck portion 2b at -a lower temperature level than any ofthey restricted cross-section portions 2c, 2c as long as the currentwhich flows through the fuse is of lesser magnitude than a shortcircuitcurrent. The fact that Ithe rate of heat flow from neck 2b to cap 6a mayWidely vary with ambient temperature conditions has, therefore, nobearing upon the operation of the fuse.

Where high current carrying capacity is required which may be in theorder of many hundreds of amperes or of the order of thousands ofamperes, a plurality of fuse units of the type shown in Figs. 1 and 2may be arranged in parallel -in the circuit to be protected andintegrated into one single self-contained structural unit. Such acomposite fuse may have two terminal elements at opposite ends thereofto which a plurality of links may be directly secured so as to be inimmediate contact with the terminal elements. However, this type ofdesign has not been found as desirable for embodying the inventioninicompositeziiigh current-carrying capacity current-limiting 1 fuses asthe structure shown in Figs;.4- and .5, particularly if 'the'circuitvoltage is relatively high, e. g. in the'order of 1000 volts. Figs.` 4and 5 show a composite fuse structure lhaving a very- 'high'current-carrying capacity particularlydes-igned andy Iadapted Yto limitthe peak of the-let-through currentl well below the order to whichthisvcouldy beachieved with any prior art current-limiting power fuse.

Referringfnow to Figs. 4` and 5, reference numerals-.mand 8b -have beenapplied to two metal blocks-which are preferablymade of copper and are`provided with blade contactsa'and 9b inn tended .-.for vinsertion into afuse holder (not shown).. Metal blocks -8a and- 8b are provided with aplurality of pairs lof substantially cylindrical recesses-|011,Ib;.l.0a., Inb.; |0a., lllb\"; Illaf, |0b; I0a".", I0b.. The fusecomprises a plurality of fuse units whichtare. identical..with.-the fuseuni-tsho'wnin Figs. .1.and'2 and which rare,- therefore, only. more orless: diagram: matically illustratedin Figs.. 4.and.A 5-. .Referencenumeral I has been.. applied to. .the tubular cas-.f ingsfof each. ofsaid plurality ..of^.f.use.. units and reference numerals.. 6a; 6h. have.been` applied .to the-pair oftermi-nal `caps..of .whic'hfone is.arranged..a.t.each. end..of each. casing.- I.... Each. of said. pair.of terminal capsid; `-(ibis inserted under pressure into. one-of .saidplurality of. recesses Illa.. -|051 .|0a.,.. .llllfr Illaf., I0'b"; etc.and tightly. held thereunto provide for' minimum resistancetocurrentand.heatow. The pulveru-` lent cooling. substance |.I,le.g.,...quartz sand, is arranged in thespaces. formedY between casing's I.llntercasing` ll'er. Hf is a safety means. in casethatbneofltha casingsl of .the fuse.` units should burst under the.pressure developed.therein .or incase. that hotgases shoulllleak `out of .the casings vLwithoutiimpairing, the same.j The inT sulating shield. l2`isarrangedibetween. the .blocks 8a! and'. 8b and. encloses the' fuse unitswhich are.arranged between blocksla and` 8b andthe sand ller. Ilarranged in thev spaces formed b/e-. tween the vfuse units, Each casing.l accommodates a fuse link 2 and a.lin`k.`surrounding .inertare-extinguishing ller 3, preferably clean quartz sand.. Each link has aplurality of serially .related portions. 2c, 2c. of. reducedcross-section adapted to determine theminimum fusing` current of thelink. Each link has at least one .additional neck"portion'..2b' ofsmaller cross-section than .said reducedfcross-section portions 2'c' 2c:Theneck'portions 2b are arrangedimme'diately adjacent; the terminalcaps6a' and theterminal caps -6a areA under considerable. pressure in.immediate vcontactv with the blo'ck"8a." Henceblock 8a: will tend* to:considerably cool the: neck pore tions 2b during 'the normal operationof the'fuse and` duringtimes when the" fuse' is subiejcted to overload'currents which have a relatively Y'small rate of" rise. 'Because-oi."the combined-cooling action' of theV caps 'Sa and the block azthe neckszbremainnormally at'a lower temperature than each'of the seriallyrelatedportionscj Zeofre: ducedy crosse-section,asA clearlyshown in Figs: 'Haland '7(b). I

Fig'. 9A shows two maximum let-through" current versus availableshort=circuitcurrent char; acteristics. The maximum let-through current.is measuredin'peak. amperes andthe'aval'abl'e shortacircuit currentin;R.'M. S. amperes... Both the abscissaeand the. ordinates'are drawn ina logarithmic scale. Theiupper curve refer`s. to..a prior .rantlmulti'pleilink ...current.-limitii'lg..A fuseeof a given prototypesimilarftotheiuse shownrin Fig; 4-. but lacking any let-throughpcurrentlimiting. neck means. The-fuse is rated -at 200 aina peres. The lowercurve refers to la fuse-of the same prototyperated at 200am-peres andgener.- allyN identical with-the .fuse to which the Aupper curve .frefers. "except: for the provisionof. letthrough current-limitingneck-portions in.the natureof the neckv portions. 2b shown in Figs. 1and 4. While. thepeakb of the let-through cur-, rent ofthe prior artfuseis way above 3000 peak amperes,. if the. available. short-circuit.current is'2.000 R. M.. S'. amperes,` the peak let-through current'l ofthe.l same fuse, upon. being modified according to this invention, .isreduced to the orderof 1500 amperes. Forthe fuse which is modifiedaccording to .this invention, the reduc. tion ofthe peaklet-throughcurrent holdstrue throughoutthe.- entire .range of availableshortcircuit. currents. If the. 'available lshort-circuit currents-is`100,000 R.. M.. S. amperes, the peak of .the let-through-current is Wayabove 40,000 peak. amperes forv the prior art structureand merely afraction of this for the structure Vvaccordingto this invention.

For low voltage circuits the ratio of peakcurrentto total. R. .M S.current-including a D. C. component-is in the order. of 1.6.8. The uppercurve in Fig. 9 shows that at available short-circuit currents of 2000R.M. S. amperes and above the. ise permits. the' entire availableshortfcir-4 cuit current.-.whose.peak,.on basis of the above1'.'68.ratio' is 3360 amperes-fto pass. The current-limiting action ofthe fusabecomes. effective only at considerably hig-her availableIshort-circuit currents. The lower curve of Fig.` 9` shows that the peakofthev let-through-current` may be limited at theoccurrenceof"short-circuit currents of relatively small magnitude at which nocurrentflimiting action takes .place in the prior art current-limitingfuse represented by the upper curve.

Fig; 10 shows the. need'. of using fuses having, dierent'. ratings Vforachieving .selectivityv when applying prior art current-limiting fuses;

Figfll shows'that. four' different fuses accordingl to Athis inventionkwhich vhave the .same rating operate selectively if. the fault currentisA inexcess of" ofythe rated current. In other'wcrds, by varying'the'let-through current a group' offlusesmay .be obtained which permitcascading andlimitation of interruption. to the faultedsection oftli'ecircuit.

InFig'Il l2fthe upper curve refers to a priorart currentelimiting fuse'and the lov/.er curve to the samciusewith'an addedneck portion whichdetermines4 the let-through-current; The upper curve in Fig; l2is one ofthe. familyshown in Fig. .10and` the lower curvev oneofv the familyshown in '.Figlllll. The; term let-through current applied to theabscissa of Fig. 9 refers to the current which the.' fuse permits topass. and in-. cludes load' `and overload currents well.` below thecurrent range at' which the. current-limiting. action occurs.

Having. disclosed several preferred". embodiments of' my. invention,.itis desiredY that the same be' not limited t'o the particular structuresdisclosed. ltiwill beA obvious to any person skilled inthe art that manymodifications andehanges may berna/.ie without departing from the broadspirit and' scope of myinvention... Therefore it is desired thattheinvention. .be interpreted .as broadlyl as. possible '.andthat itVbel-limited only asrequirezi bythe .priorstate .of the. art..

I claim as my invention:

1. A current-limiting fuse comprising a pair of link sections eachconsisting of a unitary strip of sheet metal, and each having a reduced.crosssecton portion of different cross-sectional area, one of said pairof link sections having a reduced cross-section portion of relativelylarge area being adapted when surrounded by a pulverulentarc-extinguishing material to have a required current-carrying capacitybut having an excessive let-through current, the other of said pair oflink `sections having a reduced cross-section portion of relativelysmall area and a smaller let-through current than said one of said pairof link sections, and cooling means associated with said other of saidpair of link sections suiciently effective tokeep the overload currentcarrying capacity thereof above the overload current carrying capacityof said one o said pair of link sections, said pair of link sectionsbeing serially arranged in a common pulverulent nller nlled casing tocombine required current-carrying capacity and required let-throughcurrent characteristic.

2. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and surrounded by a pulverulent arc-quenching filler, saidfuse link having a plurality oi substantially identical serially relatedportions of reduced cross-section and at least one serially related neckportion of smaller cross-section than said reduced cross-sectionportions, .the geometry of said reduced cross-section portions and ofthe portions of said link immediately adjacent said reducedcross-section portions and the geometry of said neck portion and of theportions of said link immediately adjacent said neck portion beingdetermined in such a way that the rise in temperature at said reducedcross-section portions is more rapid than at said neck portion if thenow of current in said link is in the nature of a relatively smallprotracted overload and that the rise in temperature at that neckportion is more rapid than at said reduced cross-section portions if theflow of current in said link is in the nature of a shortcircuit current.

3. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and surrounded by a pulverulent arc-quenching iiller, y

said fuse link having a plurality of portions of reduced cross-sectionand a plurality of portions of relatively large cross-sectionintermediate said reduced cross-section portions, the length of eachsaid relatively larger cross-section portions being a multiple of thelength of said reduced cross-section portions and each said reducedcross-section portions consisting of a pair of conductive elementsforming separate parallel current paths, said fuse link further havingat least one neck portion of smaller cross-section than said reducedcross-section portions, the dimensions of each constituent part of saidlink being determined in such a Way that the ratio of the rate of heatgeneration to the rate of heat dissipation is larger for each saidreduced cross-section portions than for said neck portion on theoccurrence of overcurren'ts having a relatively small rate of rise andlarger for said neck portion than said reduced cross-section portions onthe occurrence of overcurrents in the nature of short-circuit currents.

4. A current-limiting fuse comprising a casing, a fuse link of silvermounted in said casing and surrounded by quartz sand, said link having16 a pluraliiy of substantially identical circular periorations arrangedalong the center line thereof to form a. plurality of portions ofreduced cross-section, the spacing between said perforations exceedingthe length thereof but being sufdciently small to cause dissipation ofsubstantia-ily the entire perforated length of Vsaid link on theoccurrence of currents in the order `of minimum fusing current, saidfuse link/further having at least one neck portion formed by a pair oflateral notches and smaller in crossseetion than each said reducedcross-section portions; the dimensions of each constituent part of saidvlink being determined in such a Way the ratio of the rate of heatgeneration to the rate ci heat dissipation is smaller for each saidreduced cross-section portions than ior said neck portion on theoccurrence of overcurrents having a relatively rapid rate of rise.

5. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and surrounded by a pulverulent arc-quenching filler, saidfuse link having a plurality of serially related portions of reducedcross-section and at least one serially related additional portion ofsmaller cross-section than said reduced crossesection portions, thegeometry of said reduced cross-section portions and of said additionalportion and the geometry of the portions of said link immediatelyadjacent said reduced cross-section portions and immediately adjacentsaid additional portion being determined in such a Way that the ratio ofthe rate of heat generation at each said reduced cross-section portionsto the rate of heat flow away from each said reduced cross-section,portions to the portions of said link immediately adjacent said reducedcross-section portions exceeds the ratio of the rate of heat generationat said additional portion to the rate of heat flow away from saidadditional portion to the portions of said link immediately adjacentsaid additional portion on the occurrence of overcurrents having arelatively slow rate of rise, while said first menf tioned ratio issmaller than said last mentioned ratio on the occurrence of overcurrentshaving a relatively high rate of rise.

6. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and surrounded by a pulverulent arc-quenching nller, saidfuse link having a plurality of substantially identical perforationsarranged alongfthe center line thereof to form a plurality of seriallyrelated portions of reduced cross-section, the spacing of said reducedcross-section portions being sufficiently narrow to cause dissipation ofthe entire perforated length of said link on the occurrence ofrelatively small overcurrents, said fuse link having at least oneserially related neck portion of smaller cross-section than each saidreduced cross-section portions,said neck portion having at least thesame current-carrying capacity as said reduced cross-section portions,and said neck portion being adapted to rise more rapidly in temperaturethan said reduced cross-section portions on the occurrence ofovercurrents of short-circuit current proportions.

'7. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and surrounded by an `arc-quenching ller, said fuse linkhaving a plurality of serially related substantially identical portionsof reduced cross-section adapted to fuse substantially simultaneously atthe occurrence of small protracted overloads to distribute the thermaldutyof said ller along a substantial portion of the length of said link,said 17 link further having an additional serially .related portionofreduced cross-section adapted to carry higher currents than said reducedcross-section portions and .to initiate arcing more rapidly than saidreduced cross-section portions if .the rate of rise of current isrelatively high.

8. A currentflimiting fuse comprising a casing, a fuse link mounted insaid casing and sur-.- rounded by a pulverulent arc-quenching nller,said fuse link including a plurality of serially related portions ofreduced crossrsection having a predetermined ratio of heat generation toheat dissipation and at least one additional portion of smallercross-section than each of said plurality of reduced cross-sectionportions, said additional portion having a smaller ratio of heatgeneration to heat dissipation than each of said plurality of reducedcross-section portions whereby said plu,- rality of reducedcross-section portions is .caused to fuse initially at relatively smallvoverloads and whereby said additional ,portion is caused to fuseinitially at fault currents ofk short-circuit Ycurrent proportions.

9. A current-limiting fuse comprising a tubular casing, a fuse linkmounted in said casing and surrounded along a preponderant portion ofthe length 'thereof `by an inert pulverulent arc.- quenching filler,said fuse link having at least one portion of reduced cross-sectionproportioned to initiate fusing of said link at the occurrence .ofrelatively small protracted overloads, said link having at least oneadditional portion of smaller cross-section than said reducedcross-sectionportion, the ratio of the cross-sectional area .of saidreduced cross-section portion to the Vcross-serif tional area of saidadditional portion being of the order of 4:1, and cooling meansassociated with said additional portion -to impart to said additionalportion a higher current-carrying vability than the ,current-.carryingability .of said portion of reduced cross-section.

l0. A .current-limiting fuse .comprising a casing, a pair `of terminalelements, one ateach end of said casing, a fuse link interconnectingsaid terminal elements, mounted in said .casing and surrounded by apulverulent arc-quenching quartz filler, .said link having a .pluralityof `seri-- -ally related substantially identical portions .of re-V ducedcross-section adapted to cause initial fusion at the occurrence of smallprotracted overloads at one vof said reduced .cross-section portions,said link having an additional serially related portion of smallerVcross-section than said reduced cross-section portions, said additionalportion being arranged immediately -adjacent .one

of said terminal elements and cooled sufficiently effectively toincrease the current-carrying ability thereof above the current-carrying.ability of said reduced cross-section portions.

11. .Acurrent-limiting fuse comprising a tubular Vcasing of laminatedinsulating material, a fuse link of silver mounted in said casingk andsurrounded along a preponderant `portieri of the length thereof by aquartz sand ller, said fuse link having at least one portion ofreduced'cross-k section proportioned to linitiate fusing o i' saidlinkat the occurrence of relatively .small protracted overloads, said linkhaving atleast one Iportion of" smaller cross-section than said reducedycrosssection portion proportioned to determine ,the letthrough currentof said fuse, and cooling `means associated with said smallercross-,section portion to impart to said smaller cross-section 'portiona higher current-carrying ability than the .current- 18 carrying abilityof said portion of reduced crosssection.

12. A current-.limiting fuse comprising a tu.- bular casing of laminatedinsulating material, a fuse link of silver .mounted in said casing andsurrounded along a preponderant portion of the length thereof by aquartz sand filler, said fuse link having at least a first portion ofreduced cross-.section designed to determine the letthrough current ofsaid fuse, said fuse link having at least a second portion of reducedcross.- section but larger in cross-section than said :first portion,first metallic cooling means formed by said link and associated withsaid first portion, second metallic cooling means formed by said linkand `associated with said second portion, the yeffectiveness of saidfirst cooling means and the .e'ectiveness of said second cooling meansbeing related in such a Way as to cause arc initifA ation on theoccurrence of relatively high currents at said iirst Yportion and tocause arc initiation on the occurrence of relatively small pro;- tractedoverloads at said second portion.

13. A current-limiting fuse comprising a pair of spaced metal blocks, aplurality of pairs of and tightly held therein to provide for mini.-

mum resistance to -heat ow, a pulverulent cooling substance in thespaces formed between said plurality of tubular casings, an insulatingshield surrounding said pair of metal blocks and enclosing saidpulverulent cooling substance, each of said plurality of vtubularcasings accom.- modating a fuse link and a link-surrounding pulverulentarc-extinguishing filler, each link having at least a portion of reducedcross-section adapted to determine ythe `minimum fusing current of saidlink, and each link having atleast an additional portion of smallercross-section than said reduced crosssection portion, said portion Lofsmaller cross-sectionibeing arranged immediately adjacent one of saidpairs of metal blocks and adapted .on account of the cooling actionthereof to remain normally at a lower temperature than said reducedcross-section portion of said link.

14. A .current-limiting fuse comprising a pair of spaced metal blocks,ya plurality of pairs of recesses in said metal blocks, oneof each ofsaid pairs of recesses :being provided in one .of said pair of metal.blocks and the other of each `of said pairs of recesses being providedin the other 0f said vpair of metal blocks, a pluraliln]Y of tubu larcasings each having a pair of terminal caps, one on leach end thereof,each said .pair of ter-.- minal caps :being inserted under pressure in.one of ksaid plurality of pairs of recesses .and tightly held thereinto provide for minimum resistance to current and :heat now, a'pulverulentcooling substance in the spaces formed .between saidplurality .of tubular ,casings an insulating shield between ,said pair.of metal blocks and enclosing said plurality of tubular caslngs andsaid pulverulent cooling substance, eachzof said plurality of tubular.casings accommodating a fuse link and elink-surrounding pulverulentinert arcextinguishing ller, .each 4link having a plurality 19 ofserially related portions of reduced cross-section adapted to determinethe minimum fusing current of said link, and each link having at leastone additional portion of smaller cross-section than said reducedcross-section portions, said portion of smaller cross-section beingarranged immediately adjacent to one of said pair of metal blocks andadapted on account of the cooling action thereof to remain normally at alower temperature than each of said plurality of serially relatedportions of reduced cross-section.

15. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a fuse link mounted in said casing andsurrounded by a pulverulent arc-extinguishing ller, said fuse linkhaving a plurality of serially related portions of reduced cross-sectionincluding one let-through current determining portion of smallestcross-section arranged immediately adjacent to one of said terminalelements, the

length of said smallest cross-section portion being so short and thecombined heat dissipating ability of said one of said terminal elementsand of the portions of said link adjacent said smallest cross-sectionportion so high that the rate of rise in temperature at said smallestcrosssection portion is less rapid under load conditions and in the lowoverload range than at any other point of said link.

16. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a fuse link of silver mounted in saidcasing and surrounded by quartz sand, said link having a plurality ofserially related portions of reduced cross-section including alet-through current determining portion of smallest crosssectionarranged immediately adjacent to one of said terminal elements, thelength of said smallest cross-section portion being so short and thecombined heat dissipating ability of said one of said terminal elementsand of the portions of said link adjacent said smallest cross-sectionportion so high that the rate of rise in temperature at said smallestcross-section portion is less rapid than at any other point of said linkas long as the flow of current in said link is below a predeterminedcritical value.

1'7` A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a unitary fuse link mounted in said casingand surrounded by a pulverulent arc-extinguishing filler, said fuse linkhaving a plurality of serially arranged portions of reducedcross-section and a plurality of portions of relatively largecross-section intermediate said reduced crosssection portions, said fuselink further having a let-through current determining neck of smallercross-section than each of said reduced crosssection portions arrangedimmediately adjacent to one of said terminal elements, the length ofsaid neck being so short and the combined heat dissipating ability ofsaid one of said terminal elements and of the portions of said linkadjacent said neck so large that the rate o-f rise in temperature atsaid neck is less rapid under load conditions and in the low overloadrange than at any other point of said link.

18. A current limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a unitary fuse link of silver mounted insaid casing and surrounded by quartz sand, said link having a pluralityof perforations arranged along the longitudinal axis thereof, saidplurality of perforations producing a plurality of reduced cross-sectionportions each consisting of a pair eli) of conductive elements formingseparate parallel current paths, said fuse link further having alet-through current determining neck of smaller cross-section than eachof said reduced crosssection portions arranged immediately adjacent toone of said terminal elements, the length of said neck being so shortand the combined heat dissipating ability of said one of said terminalelements and of the portions of said link adjacent said neck so largethat the rate of rise in temperature at said neck is less than at anyother point of said link as long as the flow of current in said link isbelow a predetermined critical value.

19. A current-limiting fuse comprising a pair of relatively massivespaced metal blocks, means defining a plurality of pairs of recesses insaid pair of metal blocks, one of each of said pairs of recesses beingprovided in one of said pair of metal blocks and the other of each ofsaid pairs of recesses being provided in the other of said pair of metalblocks, a plurality of tubular casings each having a pair of terminalcaps, one on each end thereof, each of said tubular casings havingannular grooves therein, and each of said pair of terminal caps havingan annular flange portion depressed into a said casing groove to locksaid pair of terminal caps on one of said plurality of tubular casings,each said pair of terminal caps being inserted under pressure in one ofsaid plurality of pairs of recesses in said pair of metal blocks toprovide for minimum resistance to current and heat flow and to effectrigid connections between the pair of metal blocks, each of saidplurality of tubular casings accommodating a fuse link and alink-surrounding pulverulent inert arc-extinguishing filler, each linkhaving a plurality of serially related portions of reduced cross-sectionadapted to determine the minimum fusing current of said linkI and eachlink having at least one additional portion of smaller cross-sectionthan said reduced cross-section portion, said portion of smallercross-section being arranged immediately adjacent to one of said pair ofmetal blocks and adapted on account of the cooling action thereof toremain normally at a lower temperature than each of said plurality ofserially related portions of reduced cross-section.

20. An electric system including a pair of cascaded protective deviceshaving substantially the same current rating, at least one of saiddevices being a current-limiting fuse, said fuse comprising a casing,terminal elements on opposite ends of said casing, a fuse link mountedin said casing and surrounded by a pulverulent arc-extinguishing ller,said fuse link having a plurality of serially related portions ofreduced cross-section including one let-through current determiningportion of smallest cross-section arranged immediately adjacent to oneof said terminal elements,

the length of said smallest cross-section portion being so short and thecombined heat dissipating ability of said one of said terminal elementsand of the portions of said link adjacent said smallest cross-sectionportion so high that the rate of rise in temperature at said smallestcross-section portion is less rapid under load and moderate overloadconditions than at any` other` point of said link.

21. An electric system including a pair of cascaded protective deviceshaving substantially the same current rating, at least one of said pairof devices being atcurrent-limiting fuse, said fuse` comprising acasing, terminal elements on opposite ends of said casing, a fuse'linkof silver mounted in said casing and surrounded by quartz sand, saidlink having a plurality of serially related portions of reducedcross-section including a let-through current determining portion ofsmallest cross-section arranged immediately adjacent to one of saidterminal elements, the length of said smallest cross-section portionbeing so short and the combined heat dissipating ability of said one ofsaid terminal elements and of the portions of said link adjacent saidsmallest crosssection portion so high that the rate of rise intemperature at said smallest cross-section portion is less rapid than atany other point of said link as long as the flow of current in said linkis below a predetermined critical value.

22. An electric system including a pair oi cascaded protective deviceshaving substantially the same current rating, at least one of said pairof devices being a current-limiting fuse, said fuse comprising a casing,terminal elements on opposite ends of said casing, a unitary fuse linkmounted in said casing and surrounded by a said portions having asufficiently high heat dissipating ability to preclude fusion of saidrelatively smaller of said portions at currents of load proportions andat relatively small protracted overload currents.

25. A current-limiting fuse comprising a casing, a fuse link mounted insaid casing and at least in part surrounded by a pulverulent quartzfiller, said fuse link having a plurality of substantially identicalserially related portions of reduced cross-section, said fuse linkfurther having at least one serially related neck portion having a widthin the order of 1,@ to 1,6 of the width of said link and of smallercross-section than each of said reduced cross-section portions, saidneck portion being arranged immediately adjacent an axially outer end ofsaid link to impart to said pulverulent arc-extinguishing filler, saidfuse link having a plurality of serially arranged portions of reducedcross-section and a plurality of portions of relatively largecross-section intermediate said reduced cross-section portions, saidfuse link further having a let-through current determining neck ofsmaller cross-section than each of said reduced cross-section portionsarranged immediately adjacent to one of said terminal elements, thelength of said neck being so short and the combined heat dissipatingability of said one of said terminal elements and of the portions ofsaid link adjacent said neck so large that the rate of rise intemperature at said neck is less rapid under load and moderate overloadconditions than at any other point of said link.

23. An electric system including a pair of cascaded protective deviceshaving substantially the same current rating, at least one of said pairof devices being a current-limiting fuse, said fuse comprising a casing,terminal elements on op` reduced cross-section portions arranged immedilately adjacent to one of said terminal elements, the length of said neckbeing so short and the combined heat dissipating ability of said one ofsaid terminal elements and of the portions of said link adjacent saidneck sov large that the rate of rise in temperature at said neck is lessrapid than at any other point of said link as long as the ow of currentin said link is of smaller than a predetermined critical value.

24. A current-limiting fuse comprising a casing, a unitary fuse linkconsisting of a strip of sheet metal mounted Within said casing andsurrounded by a quartz ller, said strip of sheet metal having twoportions of reduced cross-section having diierent cross-sectional areas,the relatively larger of said portions being adapted to determine theminimum fusing current, and cooling means associated with the relativelysmaller of neck portion at least the same current-carrying capacity asany of said reduced cross-section portions, each of said reducedcross-section portions being cooled relatively ineifectively to precludethe current-carrying capacity thereof from exceeding thecurrent-carrying capacity of said neck portion.

26. A current-limiting fuse, comprising a casing, a fuse link of silvermounted Within said casing at least in part surrounded by a pulverulentquartz ller, said fuse link having a plurality of substantiallyidentical perforations arranged along the center line thereof to form aplurality of serially related portions of reduced cross-section, a tinrivet inserted into one of said perforations to reduce the fusing pointof said link at the point thereof Where said rivet is inserted therein,said link having at least one serially related neck portion having awidth in the order of 1,@ to t; of the width of said link, said neckportion being arranged immediately adjacent an axially outer end of saidlink to impart to said neck portion at least the same current carryingcapacity as any of said reduced crosssection portions, each of saidreduced cross-section portions being cooled relatively ineffectively topreclude the current-carrying capacity thereof from exceeding thecurrent-carrying capacity of said neck portion.

27. A current-limiting fuse comprising a tubular casing, a fuse linkmounted in said casing and surrounded by a pulverulent silicious lleralong a preponderant portion of the length thereof, said link havingmetallic means associated with at least one portion thereof adapted toform an alloy With the metal of which said link is made at temperaturesbelow the fusing point of said metal to initiate fusing of said link atthe occurrence ci relatively small protracted overloads, said linkhaving at least one additional portion serially related to saidalloy-forming portion having a width in the order of 1/8 to 1;@ of thewidth of said link, and cooling means associated with said additionalportion to impart to said additional portion a higher current-carryingability than the current-carrying ability of said alloy-forming portion.

FREDERICK J. KOZACKA.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,028,720 La Mar Jan. 21, 1936 2,157,907 Lohausen May 9, 1939

